Medical devices and methods of making the same

ABSTRACT

Medical devices, such as endoprostheses, and methods of making the devices are disclosed. In some embodiments, an endoprosthesis includes a first portion having a first width, and a second portion having a second width different than the first width, wherein the first and second portions have different grain sizes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of and claims priority toU.S. application Ser. No. 10/961,289, filed on Oct. 8, 2004 now U.S.Pat. No. 7,344,560, the entire contents of which are hereby incorporatedby reference.

TECHNICAL FIELD

The invention relates to medical devices, such as stents, and methods ofmaking the devices.

BACKGROUND

The body includes various passageways such as arteries, other bloodvessels, and other body lumens. These passageways sometimes becomeoccluded or weakened. For example, the passageways can be occluded by atumor, restricted by plaque, or weakened by an aneurysm. When thisoccurs, the passageway can be reopened or reinforced, or even replaced,with a medical endoprosthesis. An endoprosthesis is typically a tubularmember that is placed in a lumen in the body. Examples of endoprosthesesinclude stents, covered stents, and stent-grafts.

Endoprostheses can be delivered inside the body by a catheter thatsupports the endoprosthesis in a compacted or reduced-size form as theendoprosthesis is transported to a desired site. Upon reaching the site,the endoprosthesis is expanded, for example, so that it can contact thewalls of the lumen.

The expansion mechanism may include forcing the endoprosthesis to expandradially. For example, the expansion mechanism can include the cathetercarrying a balloon, which carries a balloon-expandable endoprosthesis.The balloon can be inflated to deform and to fix the expandedendoprosthesis at a predetermined position in contact with the lumenwall. The balloon can then be deflated, and the catheter withdrawn.

In another delivery technique, the endoprosthesis is formed of anelastic material that can be reversibly compacted and expanded, e.g.,elastically or through a material phase transition. During introductioninto the body, the endoprosthesis is restrained in a compactedcondition. Upon reaching the desired implantation site, the restraint isremoved, for example, by retracting a restraining device such as anouter sheath, enabling the endoprosthesis to self-expand by its owninternal elastic restoring force.

SUMMARY

The invention relates to medical devices, such as endoprostheses, andmethods of making the medical devices.

In one aspect, the invention features a medical device, such as anendoprosthesis, having microstructures (e.g., grain sizes) that aretailored to particular dimensions of the device. The medical device canhave enhanced mechanical performance. For example, the device can havegood fatigue resistance, good strength, and/or low recoil.

In another aspect, the invention features an endoprosthesis, including afirst portion having a first width, and a second portion having a secondwidth different than the first width, wherein the first and secondportions have different grain sizes.

Embodiments may include one or more of the following features. Theendoprosthesis has a band including the first portion. The band has anASTM E112 G value of about eight or more. The endoprosthesis includes anelongated portion extending from the band, the elongated portion havingthe second portion. The elongated portion has an ASTM E112 G value ofabout eight or less. The band is wider than the elongated portion. Theband has a grain size larger than a grain size of the elongated portion.The band has a yield strength lower than a yield strength of theelongated portion. The first and second portions have differentthicknesses. The first and second portions have different yieldstrengths. The first and second portions include a first materialselected from the group of stainless steel, a radiopaque element, and analloy including cobalt and chromium. The first portion is wider than thesecond portion, and the first portion has a grain size larger than agrain size of the second portion. The first portion has a yield strengthlower than a yield strength of the second portion, and the first widthis larger than the second width. The endoprosthesis includes a bandincluding the first portion and the second portion.

In another aspect, the invention features an endoprosthesis, including aband having a first grain size, and an elongated portion extending fromthe band, the elongated portion having a second grain size differentthan the first grain size.

Embodiments may include one or more of the following features. The firstgrain size is larger than the second grain size. The band has a widthlarger than a width of the elongated portion. The band has an ASTM E112G value of about eight or less. The elongated portion has an ASTM E112 Gvalue of about eight or more. The band has yield strength lower than ayield strength of the elongated portion. The band and the elongatedportion have the same thickness. The band and the elongated portion havedifferent thicknesses. The band and the elongated portion have the samecomposition.

In another aspect, the invention features an endoprosthesis, including afirst portion having a first width, and a second portion having a secondwidth different than the first width, wherein the first and secondportions have different yield strengths.

Embodiments may include one or more of the following features. Theendoprosthesis includes a band including the first portion, and anelongated portion extending from the band and including the secondportion, wherein the band has a yield strength lower than a yieldstrength of the elongated portion. The first width is larger than thesecond width. The first and second portions have different thicknesses.The first and second portions have the same composition. Theendoprosthesis includes a band including the first portion and secondportion.

In another aspect, the invention features a method of making anendoprosthesis, including forming a first portion of the endoprosthesisto have a first grain size, and forming a second portion of theendoprosthesis to have a second grain size different than the firstgrain size.

Embodiments may include one or more of the following features. Themethod further includes masking the endoprosthesis. The method furtherincludes contacting the endoprosthesis with a laser beam. The methodincludes subjecting the first and second portions to different heattreatments. The endoprosthesis includes a band having the first portion,and an elongated portion extending from the band and having the secondportion, the first grain size is larger than the second grain size, andthe first portion has yield strength lower than a yield strength of thesecond portion.

In another aspect, the invention features a method of making anendoprosthesis, including forming a first portion of the endoprosthesisto have a first yield strength, and forming a second portion of theendoprosthesis to have a second yield strength different than the firstyield strength.

Embodiments may include one or more of the following features. Themethod further includes masking the endoprosthesis. The method furtherincludes contacting the endoprosthesis with a laser beam. The methodincludes subjecting the first and second portions to different heattreatments. The endoprosthesis includes a band having the first portion,and an elongated portion extending from the band and having the secondportion, and the first yield strength is less than the second yieldstrength.

In another aspect, the invention features a medical device including oneor more relatively coarse grain portions and one or more relatively finegrain portions. The medical device can be, for example, an orthopedicimplant (such as a hip stem), a guidewire, or a hypotube.

Other aspects, features, and advantages will be apparent from thedescription of the preferred embodiments thereof and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of an embodiment of an expanded stent.

FIG. 2 is a detailed view of the stent of the FIG. 1.

FIG. 3 is a flow chart of an embodiment of a method of making a stent.

FIGS. 4A, 4B, and 4C illustrate an embodiment of a method of masking atube.

FIG. 5 is a lateral view of an embodiment of a hip stem.

FIGS. 6A and 6B are lateral views of embodiments of guide wires.

DETAILED DESCRIPTION

Referring to FIG. 1, a stent 20 has the form of a tubular member definedby a plurality of bands 22 and a plurality of connectors 24 that extendbetween and connect adjacent bands. During use, bands 22 are expandedfrom an initial, small diameter to a larger diameter to contact stent 20against a wall of a vessel, thereby maintaining the patency of thevessel. Connectors 24 provide stent 20 with flexibility andconformability so that the stent can adapt to the contours of thevessel. Examples of stents are described in Burmeister et al., U.S. Pat.No. 6,451,052, and exemplified by the NIR® stent (Boston ScientificCorp.).

Referring to FIG. 2, bands 22 and connectors 24 have different shapesand dimensions. As shown, the widths of the bands (W_(b)) are largerthan the widths of the connectors (W_(c)). The larger widths (W_(b))provide bands 22 with radial strength to support the vessel, and thesmaller widths (W_(c)) allow connectors 24 to flex and to conform to thevessel. Connectors 24 are bent, having a first portion 27 that is notstraight or collinear with a second portion 29. The bent shape ofconnectors 24 allows them to accommodate strain during expansion ofstent 20. In other embodiments, connectors 24 include one or more curvedportions, examples of which are described in U.S. Pat. Nos. 6,656,220;6,629,994; and 6,616,689. As shown, bands 22 include a plurality ofconnected polygons, but other embodiments, such as sinusoidal waves orzigzag waves, can be used.

In addition, bands 22 and connectors 24 also have differentmicrostructures. As shown, bands 22 and connectors 24 have differentgrain sizes, with the grains in the bands being larger than the grainsin the connectors. As a result, connectors 24 have a higher yieldstrength that the yield strength of bands 22, since grain size istypically inversely related to yield strength. The high yield strengthof connectors 24 allows them to have small cross-sectional sizes, whichallows them to easily deform so that stent 20 can conform well to avessel that is not straight. The yield strength and the section size arebalanced to allow connectors 24 to easily deform while remainingresistant to fracture. In comparison, the low yield strength of bands 22reduces elastic recoil when stent 20 is crimped to a delivery system andduring in vivo expansion. The yield strength and the section size ofbands 22 are balanced to provide good resistance to radial compressionand to control elastic recoil.

Without wishing to be bound by theory, it is believed that stent 20 canexperience relatively high levels of stress during use. For example,stent 20 can be bent as it tracks through a tortuous vessel duringdelivery, as it is expanded, and/or when it is placed in a curvedvessel. After implantation, stent 20 can also experience stress frommovement cause by a beating heart or by the subject's breathing. Thestress can strain the relatively narrow connectors 24 and fracture theconnectors. A fractured connector can provide surfaces that disruptblood flow and/or provide sites on which blood can aggregate andundesirably lead to blood clotting or thrombosis in the vessel. Byforming stent 20 with enhanced microstructures, and therefore enhancedmechanical properties, connectors 24 can tolerate the stress that canlead to fracture, while still being easily deformable. At the same time,bands 22 are able to have good radial strength to support the vessel.

As used herein, a band 22 refers to a portion of a stent that extendscircumferentially about the stent. The band can extend completely aboutthe circumference of a stent, for example, such that the ends of theband are joined, or the band can extend partially about thecircumference. The band can extend substantially linearly ornonlinearly, for example, in an undulating pattern or a zigzag patternas shown in FIG. 1. In some embodiments, bands 22 are connected togetherby integrally formed connectors that extend between and transversely tothe bands. Band 22 can have a width (W_(b)) ranging from about 0.0010inch to about 0.0075 inch. Particular widths of band 22 can be afunction of, for example, the material(s) in stent 20, the type of stent(e.g., balloon-expandable or self-expandable), and/or the desiredperformance. For example, a stent including 316L stainless steel canhave a band width (W_(b)) of from about 0.0025 inch to about 0.0075inch; a stent including an alloy of 10-60 weight percent and 316Lstainless steel constituents (PERSS®) can have a band width (W_(b)) offrom about 0.0015 inch to about 0.0070 inch; and a stent including aFe—Co—Cr—Ni alloy (such as Elgiloy, MP35N or L605) can have a band width(W_(b)) of from about 0.0010 inch to about 0.0065 inch; and a stentincluding niobium alloyed with about 1-10 weight percent zirconium,about 1-70 weight percent tantalum, or about 1-10 weight percenttungsten can have a band width (W_(b)) of from about 0.0030 inch toabout 0.0075 inch.

In some embodiments, band 22 has at least nine grains per unit area. Forexample, per unit area, band 22 can have at least twelve grains, atleast sixteen grains, at least 20 grains, at least 25 grains, at least36 grains, or higher. As used herein, a unit area is the product of thewidth (W_(b)) and thickness (T_(b)) of band 22. The number of grains isan average number of grains taken over a substantial number (e.g., 20 ormore) of cross sections of band 22.

Alternatively or additionally, the grain structure of band 22 can beexpressed in terms of an average grain size (e.g., diameter). Table 1shows how the average grain size (diameter) for four band widths (W_(b))(0.10-0.25 mm) can be related to the number of grains per unit area.

TABLE 1 Band Width (W_(b)) Unit Area Grain/Unit Area Avg. Grain andThickness (W_(b) × T_(b)) (9/T², ASTM Diameter (T_(b)) (mm) (mm²)grain/mm²) E112 G (microns) 0.254, 0.127 0.032258 279 5 64 0.204, 0.1020.020808 433 6 45 0.152, 0.076 0.011552 779 7 32 0.102, 0.051 0.0052021730 8 20

As indicated above, the unit area is determined by multiplying the widthby the thickness of a band. The number of grains per unit area (in thisexample, nine grains/unit area) can then be converted to an ASTM E112 Gvalue. (See ASTM E112 Table 4. Grain Size Relationships Computed forUniform, Randomly Oriented, Equiaxed Grains.) The average grain diametercan then be determined from ASTM E112 G value, which is inverselyproportional to the average grain diameter. (See ASTM E112.) In someembodiments, band 22 has an average ASTM E112 G value of about eight orless. The average grain diameter can range from about 20 microns toabout 64 microns. For example, the average grain diameter can be equalto or less than about 64, about 60, about 56, about 52, about 48, about44, about 40, about 36, about 32, about 28, or about 24 microns; and/orgreater than or equal to about 20, about 24, about 28, about 32, about36, about 40, about 44, about 48, about 52, about 56, or about 60microns. In embodiments in which bands 22 include one or more refractorymetals, such as niobium, tantalum, or tungsten, the grain size can befine to reduce brittleness. The grain size can be, for example, lessthan about 32 microns, e.g., less than about 28 or 24 microns.

The microstructure (e.g., grain size) of bands 22 in turn can affect themechanical properties of the bands. In some embodiments, bands 22 haveyield strengths of from about 15 ksi to about 70 ksi. The relatively lowyield strength (e.g., compared to the yield strength of connectors 24described below) allows bands 22 to be plastically deformed duringcrimping of stent and during expansion of the stent with low recoil. Theyield strength can be greater than or equal to about 15, about 20, about25, about 30, about 35, about 40, about 45, about 50, about 55, about60, or about 65 ksi; and/or less than or equal to about 70, about 65,about 60, about 55, about 50, about 45, about 40, about 35, about 30,about 25, or about 20 ksi. In certain embodiments, bands 22 including analloy including cobalt can have a yield strength from about 40-60 ksi,bands including a refractory metal (such as niobium or tantalum) canhave a yield strength from about 15-30 ksi, and bands including titaniummetal can have a yield strength from about 25-40 ksi.

Similar to bands 22, connectors 24 are also tailored to providepredetermined properties and performance. As used herein, a connector 24refers to a portion of a stent that extends from a band of the stent,for example, from a first band to an adjacent second band along thelength of the stent. The connector can include one strut or a pluralityof struts. The connector can extend linearly (e.g., parallel to thelongitudinal axis of the stent) or nonlinearly, for example, in anundulating patter or zigzag pattern. Connector 24 can have a width(W_(c)) ranging from about 0.030 mm to about 0.200 mm. Particular widthsof connector 24 can be a function of, for example, the material(s) instent 20, the type of stent (e.g., balloon-expandable orself-expandable), and/or the desired performance. For example, a stentincluding 316L stainless steel can have a connector width (W_(c)) offrom about 0.05 mm to about 0.12 mm; a stent including a PERSS® alloycan have a band width (W_(c)) of from about 0.03 mm to about 0.10 mm; astent including an alloy having chromium and cobalt can have a bandwidth (W_(c)) of from about 0.02 mm to about 0.08 mm; a stent includinga refractory metal can have a band width (W_(c)) of from about 0.08 mmto about 0.20 mm; and a stent including an alloy having titanium canhave a band width (W_(c)) of from about 0.03 mm to about 0.15 mm.

As with band 22, in some embodiments, connector 24 has at least ninegrains per unit area. For example, per unit area, connector 24 can haveat least twelve grains, at least sixteen grains, at least 20 grains, atleast 25 grains, at least 36 grains, or higher. Here, a unit area is theproduct of the width (W_(c)) and thickness (T_(c)) of connector 24 (FIG.4). The number of grains is an average number of grains taken over asubstantial number (e.g., 20 or more) of cross sections of connector 24.

Alternatively or additionally, the grain structure of connector 24 canbe expressed in terms of an average grain size (e.g., diameter), whichcan be determined as described above but using W_(c). Since connector 24is narrower (and/or thinner) than band 22 in some embodiments, the grainsize of the connector is smaller than the grain size of the band, e.g.,to provide at least nine grains per unit area. In some embodiments,connector 24 has an average ASTM E112 G value of about eight or more,including nanometer size grains that are off of the ASTM E112 G scale.The average grain diameter can range from about 30 microns to about 0.01microns (10 nanometers). For example, the average grain diameter can beequal to or less than about 30, about 25, about 20, about 15, about 10,about 5, about 1, about 0.50, about 0.25, about 0.10, or about 0.05micron; and/or greater than or equal to about 0.01, about 0.05, about0.10, about 0.25, about 0.50, about 1, about 5, about 10, 1 about 5,about 20, or about 25 microns. In certain embodiments, the grain sizecan be from about 0.1 to about 20 microns for connectors 24 including astainless steel, from about 1 to about 30 microns for connectorsincluding an alloy having cobalt, from about 10 to about 20 microns forconnectors including a refractory metal, and from about 0.1 to about 20microns for connectors including an alloy having titanium.

The microstructure (e.g., grain size) of connectors 24 in turn canaffect the mechanical properties of the connectors. For example, a finegrain size can result in a high yield strength, which in turn allows theconnector to be made thin and flexible. In some embodiments, connectors24 have yield strengths of from about 35 ksi to about 100 ksi. Therelatively high yield strength (e.g., compared to the yield strength ofbands 22 described above) allows connectors 24 to resist fatigue failureduring delivery of stent 20 and after stent placement. The yieldstrength can be greater than or equal to about 35, about 40, about 45,about 50, about 55, about 60, about 65, about 70, about 75, about 80,about 85, about 90, or about 95 ksi; and/or less than or equal to about100, about 95, about 90, about 85, about 80, about 75, about 70, about65, about 60, about 55, about 50, about 45, or about 40 ksi. In someembodiments, the yield strength can be from about 45 to about 90 ksi forconnectors including stainless steel, from about 50 to about 100 ksi forconnectors including an alloy having cobalt, from about 35 to about 60ksi for connectors including a refractory metal, and from about 40 toabout 80 ksi for connectors including an alloy having titanium.

Intermediate portions between bands 22 and connectors 24 (e.g., portion25, FIG. 2) can have microstructures intermediate that of the bands andthe connectors. In some embodiments, the intermediate portions include agradient of microstructures that includes coarse grains (e.g., near thebands) transitioning to fine grains (e.g., near the connectors). Thegradient reduces an abrupt change that can be susceptible to stress andfracture. The intermediate portions can be formed, for example, bytapering a thermal barrier or mask, as described below. In someembodiments, the intermediate portions can extend over lengths of fromabout 10 to 200 microns or about 5 to 20 times the average graindiameter of connector 24. For example, the intermediate portion betweena band with 32 micron grains to a connector with 10 micron grains canextend over a length of 50 microns. One or more intermediate portionscan be located away from the intersection of a connector 24 and a band22, such as within the band, to reduce stress concentrations in thesmall connector and at the change in width from the connector to band.

Bands 22 and connectors 24 can include (e.g., be manufactured from) oneor more biocompatible materials with mechanical properties so that stent20 can be compacted, and subsequently expanded to support a vessel. Insome embodiments, stent 20 can have an ultimate tensile strength (UTS)of about 20-150 ksi, greater than about 15% elongation to failure, and amodulus of elasticity of about 10-60 msi. When stent 20 is expanded, thematerial can be stretched to strains on the order of about 0.3. Examplesof “structural” materials that provide good mechanical properties and/orbiocompatibility include, for example, stainless steel (e.g., 316L and304L stainless steel, and PERSS®), Nitinol (a nickel-titanium alloy),Elgiloy, L605 alloys, MP35N, Ti-6Al-4V, Ti-50Ta, Ti-10Ir, Nb-1Zr, andCo-28Cr-6Mo. Other materials include elastic biocompatible metal such asa superelastic or pseudo-elastic metal alloy, as described, for example,in Schetsky, L. McDonald, “Shape Memory Alloys”, Encyclopedia ofChemical Technology (3rd ed.), John Wiley & Sons, 1982, vol. 20. pp.726-736; and commonly assigned U.S. Ser. No. 10/346,487, filed Jan. 17,2003.

The material(s) can include one or more radiopaque materials to provideradiopacity. Examples of radiopaque materials include metallic elementshaving atomic numbers greater than 26, e.g., greater than 43. In someembodiments, the radiopaque materials have a density greater than about9.9 g/cc. In certain embodiments, the radiopaque material is relativelyabsorptive of X-rays, e.g., having a linear attenuation coefficient ofat least 25 cm⁻¹, e.g., at least 50 cm⁻¹, at 100 keV. Some radiopaquematerials include tantalum, platinum, iridium, palladium, hafnium,tungsten, gold, ruthenium, and rhenium. The radiopaque material caninclude an alloy, such as a binary, a ternary or more complex alloy,containing one or more elements listed above with one or more otherelements such as iron, nickel, cobalt, or titanium. Examples of alloysincluding one or more radiopaque materials are described in U.S.Application Publication US-2003-0018380-A1; US-2002-0144757-A1; andUS-2003-0077200-A1.

In some embodiments, stent 20 includes one or more materials thatenhance visibility by magnetic resonance imaging (MRI). Examples of MRImaterials include non-ferrous metal-alloys containing paramagneticelements (e.g., dysprosium or gadolinium) such as terbium-dysprosium,dysprosium, and gadolinium; non-ferrous metallic bands coated with anoxide or a carbide layer of dysprosium or gadolinium (e.g., Dy₂O₃ orGd₂O₃); non-ferrous metals (e.g., copper, silver, platinum, or gold)coated with a layer of superparamagnetic material, such asnanocrystalline Fe₃O₄, CoFe₂O₄, MnFe₂O₄, or MgFe₂O₄; and nanocrystallineparticles of the transition metal oxides (e.g., oxides of Fe, Co, Ni).Alternatively or in addition, stent 20 can include one or more materialshaving low magnetic susceptibility to reduce magnetic susceptibilityartifacts, which during imaging can interfere with imaging of tissue,e.g., adjacent to and/or surrounding the stent. Low magneticsusceptibility materials include tantalum, platinum, titanium, niobium,copper, and alloys containing these elements. The MRI visible materialscan be incorporated into the structural material, can serve as thestructural material, and/or be includes as a layer of stent 20.

Stent 20 can be of any desired shape and size (e.g., coronary stents,aortic stents, peripheral vascular stents, gastrointestinal stents,urology stents, and neurology stents). Depending on the application,stent 20 can have a diameter of between, for example, 1 mm to 46 mm. Incertain embodiments, a coronary stent can have an expanded diameter offrom about 2 mm to about 6 mm. In some embodiments, a peripheral stentcan have an expanded diameter of from about 5 mm to about 24 mm. Incertain embodiments, a gastrointestinal and/or urology stent can have anexpanded diameter of from about 6 mm to about 30 mm. In someembodiments, a neurology stent can have an expanded diameter of fromabout 1 mm to about 12 mm. An abdominal aortic aneurysm (AAA) stent anda thoracic aortic aneurysm (TAA) stent can have a diameter from about 20mm to about 46 mm. Stent 20 can be balloon-expandable, self-expandable,or a combination of both (e.g., U.S. Pat. No. 5,366,504).

Stent 20 can be formed by heat treating bands 22 and connectors 24differently. FIG. 3 shows a method 30 of making stent 20. As shown,method 30 includes forming a tube (step 32) that makes up the tubularmember of stent 20. The tube is subsequently cut to form bands 22 andconnectors 24 (step 34) to produce an unfinished stent. Areas of theunfinished stent affected by the cutting are subsequently removed (step36). Next, selected portions of bands 22 and/or connectors 24 are maskedin a predetermined manner to allow the bands and the connectors to beheat treated differently (step 38). The masked unfinished stent is thenheat treated, e.g., using a laser (step 40). Next, the mask is removed(step 42), and the unfinished stent is finished to form stent 20 (step44).

The tube that makes up the tubular member of stent 20 can be formedusing metallurgical techniques, such as thermomechanical processes (step32). For example, a hollow metallic member (e.g., a rod or a bar) can bedrawn through a series of dies with progressively smaller circularopenings to plastically deform the member to a targeted size and shape.In some embodiments, the plastic deformation strain hardens the member(and increases its yield strength) and elongates the grains along thelongitudinal axis of the member. The deformed member can be heat treated(e.g., annealed above the recrystallization temperature and/or hotisostatically pressed) to transform the elongated grain structure intoan initial grain structure, e.g., one including equiaxed grains. Smallor fine grains can be formed by heating the member close to therecrystallization temperature for a short time. Large or coarse grainscan be formed by heating the member at higher temperatures and/or forlonger times to promote grain growth.

Next, bands 22 and connectors 24 of stent 20 are formed, as shown, bycutting the tube (step 34). Selected portions of the tube can be removedto form bands 22 and connectors 24 by laser cutting, as described inU.S. Pat. No. 5,780,807, hereby incorporated by reference in itsentirety. In certain embodiments, during laser cutting, a liquidcarrier, such as a solvent or an oil, is flowed through the lumen of thetube. The carrier can prevent dross formed on one portion of the tubefrom re-depositing on another portion, and/or reduce formation of recastmaterial on the tube. Other methods of removing portions of the tube canbe used, such as mechanical machining (e.g., micro-machining),electrical discharge machining (EDM), and photoetching (e.g., acidphotoetching).

In some embodiments, after bands 22 and connectors 24 are formed, areasof the tube affected by the cutting operation above can be removed (step36). For example, laser machining of bands 22 and connectors 24 canleave a surface layer of melted and resolidified material and/oroxidized metal that can adversely affect the mechanical properties andperformance of stent 20. The affected areas can be removed mechanically(such as by grit blasting or honing) and/or chemically (such as byetching or electropolishing). In some embodiments, the tubular membercan be near net shape configuration after step 36 is performed.“Near-net size” means that the tube has a relatively thin envelope ofmaterial that is removed to provide a finished stent. In someembodiments, the tube is formed less than about 25% oversized, e.g.,less than about 15%, 10%, or 5% oversized.

Next, selected portions of bands 22 and connectors 24 are masked (step38). Referring to FIGS. 4A, 4B, and 4C, an embodiment of a method formasking bands 22 and connectors 24 is shown. A removable shield 46 isfirst placed on bands 22 and connectors 24 over portions that are to beexposed during heat treatment (FIG. 4A). Shield 46 can be, for example,an adhesive-backed tape; a dissolvable material (such as a carbon steelthat can be dissolved by immersion in an acid such as nitric acid, whichcan also remove certain recast material formed during manufacturing); ora material (such as gallium metal) that can be melted or sublimed duringheat treatment. Shield 46 can include a ceramic and/or a glass that canbe removed by heating the tube and allowing differential thermalexpansion to separate the shield from the tube. Alternatively or inaddition, shield 46 can be removed mechanically, such as by grinding.

Next, a mask 48 is applied over bands 22 and connectors 24 to serve asan insulative thermal barrier (FIG. 4B). Examples of materials for mask48 include ceramics (such as titanium nitride, titanium carbide, andsilicon carbide), including oxides (such as aluminum oxide, zirconiumoxide, and magnesium oxide). Mask 48 can be applied by slurry dipping,spraying, powder coating, physical vapor deposition, sputtering, and/orchemical vapor deposition. Shield 46 is then removed to expose thepreviously shielded portions 50 of bands 22 and connectors 24 (FIG. 4C).Masking bands 22 and connectors 24 allow the bands and the connectors tobe heat treated differently, as described below. Masking also allowsselected small areas of the unfinished stent to be locally andthoroughly heated without substantial heat loss because the openstructure of the unfinished stent can radiate heat.

Referring again to FIG. 3, after the unfinished stent is masked, theunfinished stent is heat treated (step 40). For example, the unfinishedstent can be heated, under vacuum or under a controlled (e.g., inert)atmosphere, in a furnace, in an induction coil, or under a heat lamp. Asshown in FIG. 4C, connector 24 is more masked than band 22. As a result,when connector 24 and band 22 are heated under the same conditions, theband experiences more heating and grain growth. In some embodiments,alternatively or additionally to covering different percentages ofsurface areas of bands 22 and connectors 24, different thicknesses ofmask 48 can be deposited to effect different heating. For example, mask48 on connectors 24 can be thicker than the mask on bands 22 to providemore insulation and therefore less heating.

Alternatively or additionally to heating as above, exposed portions 50can be locally heated so that the heat treated areas are preciselytargeted. For example, exposed portions 50 can be addressed with alaser, an electron beam, or other focal heating sources, such that theheat is conducted from exposed portions 50 to the bulk of the tube. Insome embodiments of local heating, connectors 24 can be less masked thanbands 22 to dissipate heat.

In some embodiments, bands 22 and connectors 24 are not masked prior toheat treatment. Bands 22 and connectors 24 can be heat treateddifferently, for example, by lasing the bands for longer times and/orwith more energy to produce grain growth, compared to lasing theconnectors. In embodiments in which the initial grain structure of thetube is the desired grain structure of connectors 24, only theconnectors can be masked (e.g., if not using local heating) and/or onlybands 22 are heat treated to effect grain growth.

After the unfinished stent is heat treated to form the targetedmicrostructures, mask 48 is removed (step 42). Mask 48 can be removedby, for example, grit blasting, chemical milling, and/or cryogenicfracture.

The unfinished stent is then finished to form stent 20. The unfinishedstent can be finished, for example, by electropolishing to a smoothfinish. Since the unfinished stent can be formed to near-net size,relatively little of the unfinished stent need to be removed to finishthe stent. As a result, further processing (which can damage the stent)and costly materials can be reduced. In some embodiments, about 0.0001inch of the stent material can be removed by chemical milling and/orelectropolishing to yield a stent.

In use, stent 20 can be used, e.g., delivered and expanded, using acatheter delivery system. Catheter systems are described in, forexample, Wang U.S. Pat. No. 5,195,969, Hamlin U.S. Pat. No. 5,270,086,and Raeder-Devens, U.S. Pat. No. 6,726,712. Stents and stent deliveryare also exemplified by the Radius® or Symbiot® systems, available fromBoston Scientific Scimed, Maple Grove, Minn.

While a number of embodiments have been described above, the inventionis not so limited.

In some embodiments, bands 22 and connectors 24 can have the samethickness or different thicknesses. A smaller thickness, for example,can enhance the flexibility of connectors 24.

Alternatively or additionally to masking bands 22 and connectors 24, theunfinished stent can be selectively coated with a polished andreflective coating (e.g., on the connectors) and/or a blackened coating(e.g., on the bands). The polished and reflective coating (such as gold,platinum, and/or silver) can reduce the amount of heat transferred tothe unfinished stent. The blackened coating (such as graphite) canincrease the amount of heat transferred to the unfinished stent.

In some embodiments, no masking is necessary. For example, a tube asdescribed herein can be fixtured into a laser-cutting machine. The tubecan be heat treated using a de-focused laser and computer-numericcontrol. For example, the laser can be controlled to heat the areas ofthe tube that will eventually be cut to form bands 22 at a temperaturebelow the melting point of the tube material. Heat dispersion can beaccomplished by flowing a coolant through the lumen of the tube. Afterthe heat treatment, the laser can be re-focused to cut bands 22 andconnectors 24, without removing the tube from the fixture. Connectors 24can have a higher yield strength and a smaller grain size than bands 22because the bands have been heat treated.

In some embodiments, bands 22 and/or connectors 24 can include multiplewidths and/or thicknesses. For example, a band can include a first largewidth and a second smaller width. The first large width can have amicrostructure as described above for band 22, and the second smallerwidth can have a microstructure as described above for connector 24. Aconnector having multiple widths and/or thicknesses can include similarmicrostructures.

Stent 20 can include one or more layers. For example, a stent caninclude a first “structural” layer, such as 316L stainless steel, and asecond layer of a radiopaque element. The radiopaque layer can be formedafter the heat treatment to prevent, e.g., separation due to thermalexpansion differences. Either layer can be the inner or the outer layer,and either layer or both layers can include the microstructures asdescribed above. A three-layered stent can include a layer including aradiopaque element formed between two structural layers.

Stent 20 can also be a part of a covered stent or a stent-graft. Inother embodiments, stent 20 can include and/or be attached to abiocompatible, non-porous or semi-porous polymer matrix made ofpolytetrafluoroethylene (PTFE), expanded PTFE, polyethylene, urethane,or polypropylene.

Stent 20 can include a releasable therapeutic agent, drug, or apharmaceutically active compound, such as described in U.S. Pat. No.5,674,242, U.S. Ser. No. 09/895,415, filed Jul. 2, 2001, and U.S. Ser.No. 10/232,265, filed Aug. 30, 2002. The therapeutic agents, drugs, orpharmaceutically active compounds can include, for example,anti-thrombogenic agents, antioxidants, anti-inflammatory agents,anesthetic agents, anti-coagulants, and antibiotics.

In other embodiments, the structures and methods described herein can beused to make other medical devices. For example, referring to FIG. 5, anorthopedic device (as shown, a hip stem 70) can be formed to include oneor more stiff or rigid sections 72, and one or more flexible sections 74along the length of the device (e.g., stem) to provide the device withan overall stiffness similar to that of natural bone. As shown, stem 70has a long, tapered cylindrical shape, and along its length, the stemincludes relatively large diameter rigid sections 72 alternating withadjacent, relatively small diameter flexible sections 74. Rigid sections72 can be made with a selected grain size that provides a targeted yieldstrength, and small diameter sections 74 can be made with finer grainsize to provide a higher yield strength. Stem 70 can flex in flexiblesections 74, but not yield or fracture in these sections because thehigher local yield strength can prevent plastic deformation. In someembodiments, to reduce abrupt changes between sections 72 and 74 thatcan concentrate stress, one section can be tapered to the adjacentsection, and within the taper, the grain size can transition, similar tothe intermediate portions described above. Stem 70 can be made flexible,strong, and fracture resistant. In some embodiments, sections 72 and 74can have varying dimensions along the length of stem 70 so that thesections alternate with different frequencies along the length.

As another example, guidewires can be made of strong materials that havegood radiopacity and torquability (such as cobalt alloys and stainlesssteels) and be controllably processed to provide good flexibility forenhanced trackability. For example, referring to FIG. 6A, a guidewire 80includes a plurality of coarse grain sections (e.g., bands) 82alternating with a plurality of fine grain sections 84 along the taperedlength of the guidewire. Coarse grain sections 82, which can be larger(e.g., in diameter) than sections 84, can provide guidewire 80 with atargeted stiffness. Fine grain sections 84 can allow guidewire 80 toflex to enhance trackability, while not yielding or fracturing becausethe relatively high local yield strength can prevent plasticdeformation. Similar to hip stem 70 above, the dimensions, distributionand frequency of sections 82 and 84 can vary along the length ofguidewire 80, for example, to make the distal tip 86 of the guidewireparticularly flexible. Guidewire 80 can be solid (as shown in FIG. 6A)or hollow. For example, referring to FIG. 6B, a ribbon having sections82 and 84 can be tightly wound to form a guidewire defining a lumen 86.

The following examples are illustrative and not intended to be limiting.

EXAMPLE 1

The following example illustrates a method of making a stent including astainless steel, such as 316L stainless steel.

A 316L stainless steel hollow bar can be gundrilled and machined (1.0″O.D.×0.08″ I.D., from a 1.1″ diameter bar) and can be drawn to form astent tubing with an O.D. of 0.10″ and an I.D. of 0.08″. The final coldworking operation, following the last recrystallization annealtreatment, can be performed to produce 40-60% cold work in the material.The resulting tube can have textured (elongated) grains.

The tubing can be laser machined to cut a pattern of bands with a widthof 0.13 mm and thickness of 0.06 mm, and a pattern of connectors with awidth of 0.06 mm and thickness of 0.06 mm to form an unfinished stent.Laser affected areas can be removed by chemical etching andelectropolishing.

A maskant system can be applied to the unfinished stent such that thebands can receive more heat than the connectors during an annealingtreatment. The maskant can be porous layers of iron deposited by a laseror a plasma spray. The pore size of the maskant can be larger for theband surfaces (e.g., 70% porosity) than the connector surfaces (e.g.,20% porosity). The total thickness of the maskant can be about 0.06-0.10mm on all surfaces. The unfinished stent can then be bright annealed bypassing it through a furnace with a protective hydrogen atmosphere on aconveyor belt such that the unfinished stent can be exposed to atemperature of 1050° C. for 10 minutes. Afterwards, the unfinished stentcan be cooled in protective atmosphere, and the maskant can be dissolvedaway in a nitric acid solution.

The unfinished stent can then be finished by electropolishing to a finalsize and surface finish. The finished stent can have coarser grains inthe bands than in the connectors. The average grain size of the bandscan be about 45 microns, and the average grain size of the connectorscan be about 11 microns. The yield strengths can be about 40 ksi for thebands, and about 55 ksi for the connectors.

EXAMPLE 2

The following example illustrates a method of making a stent includingan alloy of stainless steel and platinum.

A PERSS® stainless steel (Fe-30 Pt-18 Cr-9 Ni-2.63 Mo) hollow bar can begundrilled and machined (1.0″ O.D.×0.08″ I.D., from 1.1″ diameter bar)and can be drawn to form a stent tubing with an O.D. of 0.10″ and anI.D. of 0.08″. The final cold working operation, following the lastrecrystallization anneal treatment, can be performed to produce 40-60%cold work in the material. The resulting tube can include textured(elongated) grains.

The tubing can be laser machined to cut a pattern of bands with a widthof 0.13 mm and a thickness of 0.03 mm, and a pattern of connectors witha width of 0.06 mm and thickness of 0.03 mm to form an unfinished stent.Laser affected areas can be removed by chemical etching andelectropolishing.

A laser deposited maskant system can be applied to the connectors suchthat the bands can receive more heat than the connectors during anannealing treatment. The maskant deposited on the surface of theconnectors can be a continuous layer of iron that triples the thicknessof the wall of the unfinished stent wall. The added thickness can slowthe heating rate of the connectors, thereby reducing the time ofexposure at the anneal temperature. The unfinished stent can then bebright annealed by passing it through a furnace with a protectivehydrogen atmosphere on a conveyor belt such that the unfinished stentcan be exposed to a temperature of 1165° C. for 10 minutes. Afterwards,the unfinished stent can be cooled in a protective atmosphere, and themaskant can be dissolved away in a nitric acid solution.

The unfinished stent can then be finished by electropolishing to a finalsize and surface finish. The finished stent can have coarser grains inthe bands than in the connectors. The average grain size of the bandscan be about 53 microns, and the average grain size of the connectorscan be about 11 microns. The yield strengths can be about 55 ksi for thebands and about 80 ksi for the connectors.

EXAMPLE 3

The following example illustrates a method of making a stent including aCo—Cr alloy.

An L605 alloy (51Co-20Cr-10Ni-15W-3Fe-2Mn) hollow bar can be gundrilledand machined (1.0″ O.D.×0.08″ I.D., from 1.1″ diameter bar) and can bedrawn to form a stent tubing with an O.D. of 0.10″ and an I.D. of 0.08″.The final cold working operation, following the last recrystallizationanneal treatment, can be performed to produce 40-60% cold work in thematerial. The resulting tube can have textured (elongated) grains.

The tubing can be laser machined to cut a pattern of bands with a widthof 0.13 mm and thickness of 0.03 mm, and a pattern of connectors with awidth of 0.06 mm and thickness of 0.03 mm to form an unfinished stent.Laser affected areas can be removed by chemical etching andelectropolishing.

A maskant system can be applied to the unfinished stent such that thebands can receive more heat than the connectors during an annealingtreatment. The unfinished stent can be dipped in a ceramic solutioncontaining zirconia or alumina and allowed to dry. The coating on thebands can later be grit blasted away. Only the exterior surface of theunfinished stent need be grit blasted, since this exposed metal canallow sufficient heat input to cause recrystallization and grain growth.The unfinished stent can then be bright annealed by passing it through afurnace with a protective hydrogen atmosphere on a conveyor belt suchthat the unfinished stent can be exposed to a temperature of 1250° C.for 10 minutes. Afterwards, the unfinished stent can be cooled in aprotective atmosphere, and the maskant can be removed by liquid mediahoning.

The unfinished stent can be finished by electropolishing to a final sizeand surface finish. The finished stent can have coarser grains in thebands than in the connectors. The average grain size of the bands can beabout 53 microns, and the average grain size for the connectors can beabout 11 microns. The yield strengths can be about 55 ksi for the bands,and about 80 ksi for the connectors.

EXAMPLE 4

The following example illustrates a method of making a stent includingan alloy containing niobium and zirconium.

A Nb-1Zr hollow bar can be gundrilled and machined (1.0″ O.D.×0.08″I.D., from 1.1″diameter bar) and can be drawn to form a stent tubingwith an O.D. of 0.10″ and an I.D. of 0.08″. The final cold workingoperation, following the last recrystallization anneal treatment, can beperformed to produce 40-60% cold work in the material. The resultingtube can have textured (elongated) grains.

The tubing can be laser machined to cut a pattern of bands with a widthof 0.13 mm and thickness of 0.10 mm, and a pattern of connectors with awidth of 0.10 mm and thickness of 0.10 mm to form an unfinished stent.Laser affected areas can be removed by chemical etching andelectropolishing.

A maskant system can be applied to the unfinished stent such that thebands can receive more heat than the connectors during an annealingtreatment. The maskant can be a vapor deposited, highly reflective goldlayer. The exterior surfaces of the bands can first be covered withstrips of adhesive tape. The unfinished stent can then be vapordeposited with gold. The strips of tapes can be peeled off the bandssuch that the gold on the strips are removed from the unfinished stent.The unfinished stent can then be vacuum annealed by loading it into avacuum heat treat furnace chamber and programming the furnace such thatthe unfinished stent can be exposed to a temperature of 1300° C. for 30minutes. The reflective surfaces on the connectors can reduce theheating rate of the connectors such that the connectors have less timeat 1300° C. than the bands. Afterwards, the unfinished stent can becooled in a protective atmosphere, and the maskant can be dissolvedaway.

The unfinished stent can be finished by electropolishing to a final sizeand surface finish. The finished stent can have coarser grains in thebands than in the connectors. The average grain size of the bands can beabout 23 microns, and the average grain size for the connectors can beabout 8 microns. The yield strengths can be about 30 ksi for the bands,and about 50 ksi for the connectors.

All publications, references, applications, and patents referred toherein are incorporated by reference in their entirety.

Other embodiments are within the claims.

1. An endoprosthesis, comprising a tubular member defined by a pluralityof bands and a plurality of connectors that extend between and connectadjacent bands, the tubular member being configured to allow for thebands to be expanded from an initial diameter to a larger diameterduring use to maintain the patency of a vessel, wherein a plurality ofthe bands have a first median grain size and at least one of theconnectors has a second median grain size less than the first mediangrain size.
 2. The endoprosthesis of claim 1, wherein the plurality ofbands have a median width that is larger than a median width of the atleast one connector.
 3. The endoprosthesis of claim 1, wherein theplurality of bands have an ASTM E112 G value of about eight or less. 4.The endoprosthesis of claim 3, wherein the at least one connector has anASTM E112 G value of about eight or more.
 5. The endoprosthesis of claim1, wherein the at least one connector has an ASTM E112 G value of abouteight or more.
 6. The endoprosthesis of claim 1, wherein the pluralityof bands each have a yield strength lower than a yield strength of theat least one connector.
 7. The endoprosthesis of claim 1, wherein theplurality of bands and the at least one connector have the samethickness.
 8. The endoprosthesis of claim 1, wherein the band and the atleast one connector have different thicknesses.
 9. The endoprosthesis ofclaim 1, wherein the band and the at least one connector comprise thesame composition.
 10. An endoprosthesis, comprising: a plurality ofbands having a first median grain size; and at least one connectorextending between and connecting at least two adjacent bands of theplurality of bands, the at least one connector having a second mediangrain size less than the first median grain, wherein the at least oneconnector is bent.
 11. An endoprosthesis, comprising: a plurality ofbands having a first median grain size, wherein the plurality of bandseach include a plurality of connected polygons; and at least oneconnector extending between and connecting at least two adjacent bandsof the plurality of bands, the at least one connector having a secondmedian grain size less than the first median grain size.
 12. Theendoproshesis of claim 1, wherein the at least one connector has amaximum width less than a minimum width of the bands.
 13. Theendoprosthesis of claim 1, wherein the plurality of bands and the atleast one connector comprise a first material selected from the group ofstainless steel, a radiopaque element, and an alloy including cobalt andchromium.
 14. The endoprosthesis of claim 1, the plurality of bands eachinclude a first portion and a second portion.
 15. The endoprosthesis ofclaim 14, wherein the first and second portions have different yieldstrengths.
 16. The endoprosthesis of claim 14, wherein the first andsecond portions comprise a first material selected from the group ofstainless steel, a radiopaque element, and an alloy including cobalt andchromium.
 17. The endoprosthesis of claim 14, wherein the first portionis wider than the second portion, and the first portion has a grain sizelarger than a grain size of the second portion.
 18. The endoprosthesisof claim 14, wherein the first portion has a yield strength lower than ayield strength of the second portion, and the first width is larger thanthe second width.